1. Field of the Invention
The invention relates to Pt-catalyzed, addition-crosslinking silicone compositions which are self-adhesive at room temperature, to their preparation, and to the materials obtained by crosslinking. These silicone compositions of the invention develop adhesion rapidly on unpretreated substrates.
2. Background Art
The terms referred to below are understood as follows:
Adhesion between substrate and cured silicone is determined, for example, by means of shear tests and/or peel tests. A distinction is made between “cohesive and adhesive fracture behavior”, which shows the appearance of the cracking of an adhesive bond between two substrates following exposure in the aforementioned tests. In the case of adhesive cracking, the silicone layer can be separated cleanly from the substrate, whereas, in the case of a cohesive appearance, the cracking occurs exclusively in the silicone layer or in the substrate.
The expressions “untreated surface” and “unpretreated substrates” refer both to substrate surfaces which are used entirely without pretreatment and to substrate surfaces which prior to the application of the silicone composition have been cleaned to remove dust, contaminants, and grease. Any such cleaning takes place, for example, only with evaporating solvents. There is no additional treatment or priming of the substrate surfaces with silicone resin, silicone layers or silane layers before the silicone composition is applied.
The epithet “rapidly curing” is applied to those Pt-catalyzed, addition-crosslinking silicone compositions which cure after mixing in a rate of not more than 4 hours, preferably within 2 hours, and more preferably within 30 minutes, at room temperature. In the case of UV-activatable systems, “rapidly” curing silicone compositions are understood to be those which, following mixing and activation by UV light, cure in a maximum time period of 30 minutes, preferably 20 minutes and more preferably not more than 10 minutes.
By “curing” here is meant not necessarily the complete attainment of the ultimate mechanical properties. This term, instead, describes the state of attainment of cohesive properties which at least allow the material to be exposed to mechanical pressure without suffering destruction.
By “room temperature” is meant the temperature on the substrate surface or of the silicone in the 22-28° C. range.
Pt-catalyzed, addition-crosslinking silicone compositions known from the prior art typically lack sufficient adhesion to unpretreated substrates when crosslinked at room temperature. To achieve adhesion it is common to use functional propyltrialkoxysilanes as adhesion promoters. This results in adhesion to a range of substrates, subject to the proviso that a thermal treatment at a temperature of at least 70° C., preferably at least 100° C., is carried out, as described in U.S. Pat. No. 5,164,461, for example. As shown by U.S. Pat. No. 5,416,144, it is possible through thermal treatment to develop adhesion even within a short time. In the case of crosslinking at room temperature, in contrast, cohesive adhesion is achieved only after days, as described in U.S. Pat. No. 5,468,794 and U.S. Pat. No. 5,380,788.
In many applications of Pt-catalyzed silicone compositions, thermal crosslinking or aftertreatment is not possible, since the oven operations required for it entail additional operating costs and investment costs, or because the components and substrates used do not allow higher temperatures. Particularly in the case of Pt-catalyzed silicone compositions which offer rapid curing at room temperature (<30 minutes) without subsequent thermal treatment, such as two-component systems or UV-activable systems, therefore, what has been lacking to date is the possibility for rapid development of adhesion.
WO 2006/010763 A1 describes UV-activable, addition-crosslinking silicone compositions very generally for producing coatings. However, there is no information at all as to whether and, if so, how adhesion is obtained in the described compositions, or the substrates on which adhesion is achieved.
For the general area of UV-crosslinking systems, EP 0 356 075 B1 describes the use of a silane having twin trialkoxysilane functionalization in conjunction with a catalyst for a condensation reaction in a polymerization reaction entailing curing via a UV photoinitiator. Cohesive cracking is found after just a few hours. Disadvantages of the systems include the need to use two different crosslinking systems, and the presence of decomposition products following the irradiation of the photoinitiator. Particularly in applications in the area of electronic components, the decomposition products may result in problems.
U.S. Pat. No. 6,908,682 B2 describes Pt-catalyzed, UV-activatable, addition-crosslinking silicone elastomers comprising >80% of a silicone oil in the area of encapsulants, which achieve moderate pull-off forces in silicone gel compositions. The bonds in question entail weak physical and hence reversible adhesion of a gel, as shown by the figures of up to 4.0 N/100 mm on polybutyl terephthalate and polycarbonate. There is no mention, however, of how rapidly the adhesion is developed.